Review
AB5 toxins: structures and inhibitor design

https://doi.org/10.1016/S0959-440X(00)00152-4Get rights and content

Abstract

High-resolution crystal structures of AB5 toxins in their native form or in complex with a variety of ligands have led to the structure-based design and discovery of inhibitors targeting different areas of the toxins. The most significant progress is the development of highly potent multivalent ligands that block binding of the toxins to their receptors.

Introduction

AB5 toxins are a class of medically important bacterial toxins, named after their unique architecture comprising a single catalytically active A subunit and a pentamer of B subunits. The B pentamer is responsible for binding to cell-surface receptors, a function retained even in the absence of the A subunit. However, the complete AB5 holotoxin is required for the toxic effects. The class of AB5 toxins may be subdivided into families on the basis of sequence homology and catalytic activity (Fig. 1). The cholera toxin (CT) family includes CT itself, the Escherichia coli heat-labile enterotoxins (LTs) LT-I [1], [2], [3] and LT-II [4], [5], [6], and a less well characterized toxin from Campylobacter jejuni [7]. These toxins all have A subunits with ADP-ribosylation activities targeting an arginine of Gsα (the α subunit of the stimulatory trimeric G protein). CT and LT-I share 80% sequence homology in both the A and the B subunits, whereas they have lower sequence homology to LT-II. The Shiga toxin (SHT) family [8] includes a number of toxins from Shigella dysenteriae [9] and the Shiga-like toxins (SLTs) (also known as verotoxins) from certain E. coli strains (SLT-I and SLT-II) [10]. SHT and SLT-I are almost identical, with small variations in their A subunits, which are N-glycosidases. SHT and SLT-I are about 60% identical to SLT-II in both the A and the B subunits. Pertussis toxin (PT), produced by Bordetella pertussis and the causative agent of whooping cough, has very low sequence homology to the CT and SHT families of AB5 toxins, yet structural homology is preserved [11]. It is an exceptional AB5 toxin in that its five B subunits are not identical. The A subunit of PT catalyzes the ADP-ribosylation of a cysteine of Giα (the α subunit of the inhibitory trimeric G protein). The AB5 toxins have a wide range of toxic effects on human populations, from the relatively mild travelers’ diarrhea caused by LT-I to the much more serious and sometimes life-threatening diarrhea caused by Vibrio cholerae and the hemolytic uremic syndrome (‘hamburger disease’) caused by members of the SHT family. The AB5 toxins are probably responsible for over a million deaths annually worldwide and remain a severe medical problem.

The structure and function of the AB5 toxins were reviewed in detail in 1995 [12]. Since then, a large collection of structures of these toxins in a variety of forms has been obtained. These structures provide new insights into the biological functions of the toxins, as exemplified by the recently determined structure of a group II SLT mutant in complex with a receptor analog, which revealed residues critical for receptor binding specificity [13radical dot]. In addition, high-resolution structures of AB5 toxins with or without a bound ligand offer critical guidance for the structure-based design of inhibitors that can potentially be used to combat AB5-toxin-caused diseases. The great advances since 1999 in ligand design will be the main focus of this brief review.

Section snippets

New structural insights: receptor binding by Shiga-like toxins

Although members of the cholera family of toxins each contain five identical receptor-binding sites, each formed primarily by one B subunit with a minor contribution from a neighboring subunit, the SLTs are more complicated (Fig. 2). Despite the smaller size of their B subunits, SLT-I and SLT-II are found to bind gangliosides at up to three sites per B subunit. Specifically, the crystal structure of the SLT-I B pentamer in complex with a trisaccharide analog of SLT-I's natural receptor, Gb3,

Structure-based inhibitor design for AB5 toxins

So far, more than 30 high-resolution crystal structures of AB5 toxins are available, mostly for the cholera and the Shiga families of toxins, in a wide variety of apo, mutant and complex forms. They form an excellent basis for designing potent inhibitors against these toxins through a structure-based approach [17]. The mechanism of action of AB5 toxins offers several critical steps that can be targeted. First, one could design inhibitors that block the assembly of the holotoxin. Second, one

Conclusions

Many crystal structures of AB5 holotoxins or their subunits in complex with the natural receptors or synthetic inhibitors have appeared in the past few years. Such high-resolution structures provided critical insights for the design of novel toxin inhibitors through structure-based drug design and combinatorial chemistry. Particular progress has been made since early 1999 towards designing inhibitors blocking either the toxin assembly process or the cell-surface receptor recognition process.

Acknowledgements

The authors would like to thank Professor David Bundle and Bianca Hovey for providing figures, and the National Institutes of Health for financial support (AI44954 to EF, GM54618 to CLMJV and AI34501 to WGJH).

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • radical dot of special interest

  • radical dotradical dot of outstanding interest

References (49)

  • P Arya et al.

    Alpha-galactose based neoglycopeptides. Inhibition of verotoxin binding to globotriosylceramide

    Bioorg Med Chem

    (1999)
  • EA Merritt et al.

    Structural foundation for the design of receptor antagonists targeting Escherichia coli heat-labile enterotoxin

    Structure

    (1997)
  • WE Minke et al.

    Using a galactose library for exploration of a novel hydrophobic pocket in the receptor binding site of the Escherichia coli heat-labile enterotoxin

    J Biol Chem

    (1999)
  • GD Glick et al.

    Ligand recognition by influenza virus

    J Biol Chem

    (1991)
  • DT Connolly et al.

    Binding and endocytosis of cluster glycosides by rabbit hepatocytes

    J Biol Chem

    (1982)
  • R Roy

    Syntheses and some applications of chemically defined multivalent glycoconjugates

    Curr Opin Struct Biol

    (1996)
  • WGJ Hol et al.

    Structure and function of E. coli heat labile enterotoxin and cholera toxin B-pentamer

  • TK Sixma et al.

    Crystal structure of a cholera toxin related heat-labile enterotoxin from E. coli

    Nature

    (1991)
  • EA Merritt et al.

    Crystal structure of cholera toxin B-pentamer bound to receptor GM1 pentasaccharide

    Protein Sci

    (1994)
  • CL Pickett et al.

    Genetics of type IIa heat-labile enterotoxin of Escherichia coli: operon fusions, nucleotide sequence, and hybridization studies

    J Bacteriol

    (1987)
  • CL Pickett et al.

    Cloning, nucleotide sequence, and hybridization studies of the type IIb heat-labile enterotoxin gene of Escherichia coli

    J Bacteriol

    (1989)
  • F Van den Akker et al.

    Crystal structure of a new heat-labile enterotoxin, LT-IIb

    Structure

    (1996)
  • ME Fraser et al.

    Crystal structure of the holotoxin from Shigella dysenteriae at 2.5Å resolution

    Nat Struct Biol

    (1994)
  • PE Stein et al.

    Crystal structure of the cell-binding B oligomer of verotoxin-1

    Nature

    (1992)
  • Cited by (0)

    View full text